1. Field of Invention
The invention relates generally to a heat treating furnace for the annealing of workpieces. More particularly, the invention relates to a system and method for annealing a workpiece with accurate temperature control and uniformity.
2. Description of Related Art
There have been serious problems encountered in the annealing of some types of coiled material, such as light aluminum stock. Because of the requirements of the heat treating cycle and the characteristics of the material in the coil form, the conventional annealing furnaces which have been used for this task have been found to be unsatisfactory. The heat treating cycle requires that the aluminum be raised to a fairly specific temperature, such as 700xc2x0 F., and soaked at that temperature for a substantial period of time. It is important that the material not be heated higher than the target temperature because various types of deterioration occur at these elevated temperatures. Accordingly, it is the objective in the annealing furnace to heat the material as quickly as possible to the target temperature, to maintain it at that temperature for the desired soaking period, and then to cool the material as quickly as possible.
Much of the aluminum sold today by mills is in the form of large coils of stock which are to be used by sheet material fabricators. While aluminum is essentially a good conductor of heat, it has been found that the coiled aluminum presents serious problems as far as conducting heat from the exterior to the interior portions of the coil. The adjacent layers of aluminum present obstacles to conduction of heat radially through the coil. In some instances, there will be minute air spaces which effectively insulate the adjacent layers and in other cases the contact between the layers will be of such a limited nature as to inhibit the heat transfer by conduction. Because the interior of the aluminum coil is more or less insulated from the exterior, it has been found to be very difficult to raise the temperature of the entire coil equally to the desired target temperature. If the heating is performed too rapidly, the interior of the coil will lag far behind the exterior temperature.
An example of a conventional annealing furnace which raises the temperature of a plurality of coils to the desired target temperature is illustrated and described in U.S. Pat. No. 3,517,916 to Ross et al. In Ross et al., the heaters are maintained at a maximum gas temperature, Tmax, in excess of the desired anneal temperature, Tann. until a coil reaches a so-called control band. When the coil reaches the control band, ratio control is used such that the heat emitted by the heater is reduced to prevent the coil from being heated to a temperature above the anneal temperature Tann. The controller thereafter controls the atmospheric temperature to maintain a selected ratio between two temperature increments. The two temperature increments are based on (1) the increment xcex94G of the atmospheric temperature above Tann and (2) the increment xcex94W of the coil temperature below Tann. Thus, as the temperature of the coil reaches the annealing temperature Tann, the atmospheric temperature is reduced to the annealing temperature Tann.
For example, as used in Ross et al., if the annealing temperature Tann. is 700xc2x0 F., and the maximum gas temperature Tmax is 1000xc2x0 F. and the control band is set at 600xc2x0 F., a ratio R of xcex94G/xcex94W=3 is used. When the temperature of the coil reaches 600xc2x0 F., ratio control is performed to maintain the ratio R of 3. Thus, for each 1xc2x0 F. that the temperature of the coil increases, the atmospheric temperature is reduced by 3xc2x0 F. until both the coil and the air temperature are 700xc2x0 F.
However, a thermocouple for the coil giving the highest work temperature reading in Ross et al. is used for control purposes. One drawback in using the thermocouple giving the highest work temperature is that thermocouples for other coils giving a cooler work temperature require a longer period of time to reach the desired target temperature, i.e., Tann. The longer period of time is required because the atmospheric temperature is lowered before the cooler coils reach the control band. The furnace is thus operated for longer periods of time, thereby increasing furnace operating expenses.
Accordingly, the invention provides an annealing furnace system and method which can sense the temperature of a workpiece within each zone of the furnace and adjust the atmospheric temperature in each zone to place hotter air in a zone with the coldest workpiece and cooler air in zones with the hottest workpiece.
The invention separately provides an annealing furnace system and method which uses an adjustable formula for reaching a desired target temperature for each workpiece at approximately the same time.
The invention separately provides an annealing furnace system and method which adjusts the atmospheric temperature to suit the heating rate of the workpieces.
The invention separately provides an annealing furnace system and method which permits a minimal amount or no overheating of the workpieces.
The invention thus provides a system and methods for performing adjustable ratio control for a furnace with a plurality of workpieces located within a furnace and a sensor that determines the temperature of a workpiece in each zone. Adjustable ratio control is performed by controlling the temperature of the atmosphere wherein a first workpiece with a lower work temperature is heated at a higher rate than a second workpiece with a higher work temperature.